Actually a lot of those types of deals are as is, unless specified in a contract. It would be hard to determine whether or not the selling party knew about the health of the horse before hand, unless of course there is a paper trail.
Civil litigation is can be very lengthy and expensive. Our courts right now are backed up like you wouldn't believe. In the county I live in, our courts are looking at 18 mos to 2 yrs for a spot on the docket.
My advice is that if you or someone you know is thinking about pursuing a civil suit I would talk to a lawyer first and foremost to see if there is any merit at all, it could be that you could nail the seller for misrepresentation of the horse.
On a personal note: When we bought our mini bulldog, we were told that she was in perfect health, and for the most part she is. Except for the fact that she is extremely allergic to vaccinations, and now could potentially have mange. So yea.. I've spent a large amount of money on this dog that was supposed to be in perfect health. I could probably go back to the breeder and see what he could do for me, but.. I know I would get no where because like I said before, a lot of those deals are 'as is'.